Liquid crystals are generally divided into three types, nematic, smectic and cholesteric. Most liquid crystal displays utilize nematic liquid crystals. From the optical point of view, nematic liquid crystals are uniaxial and the nematic liquid crystals may be further subdivided as to whether they have positive or negative dielectric anisotropy. The dielectric anisotropy is given by the formula EQU .DELTA..epsilon. = .epsilon..sub..parallel. -.epsilon..sub..perp.
where .epsilon..sub..parallel. and .epsilon..sub..perp. are the dielectric constants as determined in the direction parallel to the axis of the molecule and perpendicular to the axis of the molecule, respectively.
The type of display principle to be used with a nematic liquid crystal depends upon whether .DELTA..epsilon. is positive or negative. Where .DELTA..epsilon. is positive, namely the liquid crystal material is of positive dielectric anisotropy, it is suitable for use in a display utilizing the FE (field effect) method. Where the anisotropy is negative, the DSM (Dynamic Scattering Mode) is used.
To utilize the FE type liquid crystals, advantage is taken of the fact that the material can be placed between transparent plates which have been unidirectionally rubbed. By orienting the plates with respect to each other so that the directions of rubbing are different, optical rotation is brought about. The FE liquid crystals are active in this respect only in the absence of an electric field. In the presence of an electric field, the molecules align themselves with the major axes thereof in the direction of the electric field. Consequently, in the presence of an electric field, the FE liquid crystals lose their optical rotatary power.
In order to transform FE liquid crystals from the optically active state to the inactive state, the voltage imposed across the material must be above a certain threshold. It is known that the threshold voltage is almost inversely proportional to the square root of the dielectric anisotropy .DELTA..epsilon.. A variety of display cells operating by the FE method are known; however, they suffer from problems with respect to operable life, temperature characteristics, the size of the threshold, etc. As an example of the difficulties encountered, the Schiff bases have been used because of the fact that they are practically colorless. However, the Schiff base liquid crystals are unstable chemically and undergo hydrolysis in the presence of minute quantities of water. Moreover, when subjected to an electric field between conductive electrodes as is the case in a display cell, the materials undergo electrochemical change, either oxidative or reductive, and deteriorate quickly.
Another type of liquid crystal material which has been used in the attempt to avoid instability is the azo type liquid crystal which has an azo coupling near the center of the molecule. However, it is deep red in color so that the legibility of display devices using such a material is poor, particularly when the ambient light level is low.
Liquid crystal material compositions suitable for use by the FE technique are obtained generally by mixing a liquid crystal material of positive dielectric anisotropy having a cyano (nitrile) group, -- C.tbd.N, at one end of the molecule with a second liquid crystal material of negative dielectric anisotropy. Unfortunately, it has not been possible to use substantial quantities of conventional liquid crystal materials having a high dielectric anisotropy in such compositions because of the fact that such compounds as have been known have high melting points. Accordingly, since the .DELTA..epsilon. for the composition cannot be raised to a high level, it has been necessary to use substantial driving voltages. Under such conditions, either a battery must be used or a booster must be present. Moreover, when such compositions have contained substantial quantities of the positive type liquid crystal, then the temperature range over which the material is in the mesomorphic phase is narrowed, even though by incorporation of such large quantities the threshold driving voltage is lowered. Consequently, such compositions are not suitable for general use in display cells since a relatively small change in the ambient temperature can result in the material losing its ability to rotate polarized light. What is needed, then, are liquid crystal materials having high positive dielectric anisotropy which can be incorporated in sufficient amount into liquid crystal material compositions such that the resultant .DELTA..epsilon. for the composition is high and the mesomorphic temperature range is wide. In addition, the mesomorphic temperature range, preferably, should include ordinary room temperature.